Problem: Which of these numbers is prime? ${11,\ 21,\ 35,\ 90,\ 93}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 11 are 1 and 11. The factors of 21 are 1, 3, 7, and 21. The factors of 35 are 1, 5, 7, and 35. The factors of 90 are 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, and 90. The factors of 93 are 1, 3, 31, and 93. Thus, 11 is a prime number.